Method and Device For Injecting Reactive Current During a Mains Supply Voltage Dip

ABSTRACT

The invention relates to a method and device for injecting current during a mains supply voltage dip. The inventive method consists in permanently monitoring the magnitude and phase of the mains supply voltage and, upon detection of a voltage dip, injecting reactive current in the affected main supply phase(s) in a manner that can vary over the duration of the voltage dip, at very short intervals, proportionally to the magnitude of the dip, thereby absorbing the necessary active current. The device comprises a voltage measurement switch, a transformer (low voltage/medium voltage), a DC/AC inverter, a capacitor, a voltage dip detection circuit, and a control circuit.

FIELD OF THE INVENTION

This invention refers to a procedure and a device for injecting reactivepower onto a network to which one or various electrical power generationmachines are connected when a network voltage dip occurs, particularlyfor application at wind farms.

BACKGROUND OF THE INVENTION

On the electricity network connection at the output of a farm ofelectricity generating machines such as wind turbines, a “voltage dip”may occur, which consists of a brusque decrease in voltage as a resultof faults on the network, which causes a dip zone or constant decreasein voltage and a swell zone with increasing voltage once the fault whichcaused the dip is resolved, both of which have a determined durationdepending on the voltage dip that occurred. Due to the technicalrequirements of the network, during the voltage dip a time variableinjection of the reactive power required by the machines is needed, inorder to contribute toward re-establishing the voltage and reducing theaffects of said dip.

In this sense, the regulatory bodies for electrical networks definecurves which determine the value of the reactive power that should beinjected onto the network during the voltage dip. These curves give anreactive power value according to the depth of the detected dip.

In this technique there are known devices with which reactive power ispermanently injected at a variable setting which can be changed in timeframes of minutes or seconds.

With such devices however, the reactive power to be injected cannot beadapted to the variable network voltage in very short periods of time.

In addition, the known devices are designed with overload capacitieslimited to two or three times their nominal capacities, which makestheir application with high power economically impractical.

This invention is focussed on providing a solution to this problem.

SUMMARY OF THE INVENTION

The first aspect of this invention proposes a procedure for injectingreactive power onto a network to which one or various electrical powergeneration machines are connected when a voltage dip occurs, whichcomprises the following steps:

-   -   Permanent monitoring of the magnitude and phase of the network        voltage.    -   On detecting a mono-phase, two-phase or three-phase voltage dip,        injecting reactive power on the phase or phases of the network        in a variable manner for the time duration of the voltage dip in        proportion to its magnitude, absorbing the active power        necessary.

The procedure allows the injection of the power required by theregulatory bodies on detecting the variation of the network voltagewithin very small time intervals and is able to vary the magnitude ofthe reactive power injected, also in within very small time frames.

Secondly, this invention proposes a device for carrying out theaforementioned procedure for a medium voltage network which integrates,as principal components, the following:

-   -   A medium voltage switch.    -   A low voltage/medium voltage transformer (LV/MV).    -   A dc/ac inverter.    -   A capacitor for energy storage and device stability.    -   A circuit for detecting voltage dips on the MV network;    -   A control circuit for the group of above mentioned elements,        which comprises a circuit for detecting the voltage dip on the        MV network that, once it detects that the network voltage value        is below the setting threshold, starts up the invention device        in order for it to inject the instantaneous reactive power        required to balance the network voltage according to the        technical requirements indicated by the aforementioned curve and        for the transitory duration of the voltage dip that has        occurred.

Other characteristics and advantages of this invention become apparentin the detailed description and the accompanying figures that follow.

DESCRIPTION OF FIGURES

FIG. 1 shows a block diagram with the components that make up a deviceaccording to this invention.

FIG. 2 shows a graph of the reactive power that must be injected ontothe network during the transitory duration of the voltage dip.

DETAILED DESCRIPTION OF THE INVENTION

The device according to the invention for a wind farm is comprised of,in a preferred embodiment, the following elements:

a) A medium voltage switch 1 for the connection of the device to themedium voltage circuit, from the output 12 of the farm to the networkconnection 14;

b) A LV/MV transformer 2 connected on its LV side to a dc/ac inverter 3and, on its MV side to the switch 1 for each phase;

c) A dc/ac inverter 3, preferably with IGBT or IGCT semiconductors, foreach phase, connected on its dc side to a capacitor 4, and on its acside to the LV primary of the transformer 2;

d) A dc capacitor 4 connected to the power circuit of the inverter 3 onits dc side;

e) A control circuit 20 for the group of aforementioned units, connectedto the controls 10,12,13 associated with them and which comprises thefollowing elements:

-   -   a MV voltage sensor, or voltage transformer;    -   a LV power sensor, hall effect transformer or similar;    -   a dc voltage sensor, voltage transformer or hall effect        transformer or similar;    -   a voltage dip detection circuit 10 with DSP, which detects the        voltage value below the setting threshold of the device        according to this invention;        -   a device for calculating the instantaneous reactive power to            be injected, DSP or microprocessor;        -   a device for calculating the instantaneous active power to            be absorbed;        -   a hysterisis band generator for the instantaneous reactive            power to be injected;        -   a comparison circuit for the actual power circuit readings            with those preset by the bands;        -   a circuit for generating on and off pulses for the            auto-switching power semiconductors;        -   a fibre optics coupling circuit between the described            circuit and the trigger circuits of the semiconductors;        -   a control and manoeuvre circuit for the different power            stages.

In stead of the aforementioned mono-phase components, the device may becomprised of three-phase components when an injection per phase is notrequired, with its operation similar to that described below.

When a voltage dip occurs on the MV network 14, it is characterised by azone of a brusque decrease in voltage, a dip zone of constant depth anda swell zone of increasing voltage once the fault causing the dip hasbeen cleared.

To remedy the affects of said dip on the network, the regulatory bodiesof the electrical networks define a curve (FIG. 2) on which the abscissaaxis represents the voltage T at the point of connection to the network(in terms of its relation with the nominal voltage) and the ordinateaxis represents the reactive power I (in terms of its relation with thetotal power). The date F1 indicates a normal operating situation and thedate F2 indicates a voltage dip situation in which the curve C definesthe magnitude of the reactive power that must be injected.

The voltage dip detection circuit 10, preferably with DSP, using theappropriate algorithms and based on the values of the instantaneousvoltage reading of the MV network 14, continuously calculates the peakand rms values of said voltages, and its phase, triggering the start-upprocess of the device according to this invention when it detects avoltage value below the threshold to which said device has been set.This circuit 10 is capable of detecting a dip in 1 millisecond, and cancalculate the reactive component in amplitude and phase according to therelation defined in the C curve in 2 milliseconds, injecting reactivepower some two milliseconds after the appearance of the dip on thenetwork 14.

Once the dip is detected, the control circuit 20 calculates the reactivepower to be injected based on the values of the instantaneous voltagemeasured on the network 14 by the circuit 10 based on the relationdefined on curve C, along with the instantaneous active power that needsto be absorbed from the MV network, using the transformer 2 and theinverter 3, thus maintaining the voltage of the capacitor 4 connected onthe dc side of the inverter 3. In addition, the control circuit 20comprises a manoeuvre and control circuit for the different sections ofthe power stages, in order to control the switching on and off of eachof them based on the reactive power value that needs to be injectedsequentially at each instant.

Other important characteristics of the device according to thisinvention, are as follows:

-   -   The instantaneous power handled during the duration of the        voltage dip by the device of this invention is between 20 to 30        MW, 30 times more than the admissible power of devices with        standard auto-switching elements.    -   The device needs no power source or ac/dc rectifier to function        because it takes its power from the ac network and transfers it        in a controlled manner to the capacitor connected on dc.    -   The device is capable of extracting the necessary active power        from the network in order to carry out its function even when        network voltages falls to 20% of its nominal value, in other        words when the depth of the voltage dip reaches 80%.    -   The device is able to inject variable reactive power on each        phase, and therefore acts correctly in the event of mono-phase        or two-phase faults.    -   The device is modular on each phase, therefore, in the event of        a fault on one module, the remaining modules continue to        operate.    -   The device has a sequential control, therefore only the correct        number of modules will function at each instant, based on the        intensity that must be injected onto the network at any instant,        in order to improve performance with low power and to reduce the        injected harmonic distortion.

In the embodiment we have just described, modifications can be madewithin the range defined in the following claims:

1. Device for injecting reactive power onto a MV network (14) to whichone or various electrical power generation machines are connected when avoltage dip occurs on said network (14) due to a mono-phase, two-phaseor three-phase fault, characterised in that it incorporates thefollowing elements: a) a medium voltage switch (1) on the connectionfrom the wind farm (12) to the network (14); b) a LV/MV transformer (2)on each phase; c) a dc/ac inverter (3) on each phase with IGBT or IGCTsemiconductors; d) a dc capacitor (4); e) a control circuit (20) on eachphase of the group of the aforementioned units, connected to thecontrols (10, 12, 13) associated with each of said units and whichincludes voltage and power sensors, a voltage dip detection circuit (10)with DSP, devices for calculating the instantaneous voltage on thenetwork (14) and voltage comparison circuits, so that when the circuit(10) detects a voltage dip, reactive power is injected onto the MVnetwork (14) in a variable manner at each time interval, based on themeasurements provided by the aforementioned sensors and circuits, takingthe necessary active power from the network (14) to the capacitor (4).2. Device for injecting reactive power into a MV network (14) to whichone or various electrical power generation machines are connected when avoltage dip occurs on said network (14) due to a three-phase fault,characterised in that it incorporates the following elements: a) amedium voltage switch (1) on the connection from the wind farm (12) tothe network (14); b) A LV/MV three-phase transformer (2); c) a dc/acthree-phase inverter (3) with IGBT or IGCT semiconductors; d) a dccapacitor (4); e) a control circuit (20) for the group of theaforementioned units, connected to the controls (10, 12, 13) associatedwith each of said units and which includes voltage and power sensors, avoltage dip detection circuit (10) with DSP, devices for calculating theinstantaneous voltage on the network (14) and voltage comparisoncircuits, so that when the circuit (10) detects a voltage dip, reactivepower is injected onto the MV network (14) in a variable manner at eachtime interval, based on the measurements provided by the aforementionedsensors and circuits, taking the necessary active power from the network(14) to the capacitor (4).
 3. Device for injecting reactive power onto aMV network (14) to which one or various electrical power generationmachines are connected when a voltage dip occurs on said network (14)due to a mono-phase, two-phase or three-phase fault, according to claim1, characterised in that the voltage dip detection circuit (10) detectsa voltage dip due to mono-phase, two-phase or three-phase faults in atimeframe equal to or less than 1 millisecond.
 4. Device for injectingreactive power onto a MV network (14) to which one or various electricalpower generation machines are connected when a voltage dip occurs onsaid network (14) due to a mono-phase, two-phase or three-phase fault,according to claim 1, characterised in that the control circuit (20)varies the magnitude of the reactive power injected in time intervals ofless than or equal to 20 milliseconds.
 5. Device for injecting reactivepower onto a MV network (14) to which one or various electrical powergeneration machines are connected when a voltage dip occurs on saidnetwork (14) due to a mono-phase, two-phase or three-phase fault,according to claim 1, characterised in that the device takes its powerfrom the MV network (14).
 6. Device for injecting reactive power onto aMV network (14) to which one or various electrical power generationmachines are connected when a voltage dip occurs on said network (14)due to a mono-phase, two-phase or three-phase fault, in accordance withclaim 1, characterised in that it is built in modules so that in theevent of one of the modules failing, the remaining modules can continueto operate.
 7. Device for injecting reactive power onto a MV network(14) to which one or various electrical power generation machines areconnected when a voltage dip occurs on said network (14) due to amono-phase, two-phase or three-phase fault, according to claim 1,characterised in that said machines are wind turbines.
 8. Procedure forinjecting reactive power with a device onto a network to which one orvarious electrical power generation machines are connected when avoltage dip occurs on said network due to a mono-phase, two-phase orthree-phase fault, according to claim 1, characterised in that itincludes the following steps a) Permanent monitoring of the magnitudeand phase of the network voltage b) On detecting a mono-phase, two-phaseor three-phase voltage dip, injecting reactive power on the affectedphase or phases of the network in proportion to the magnitude of thevoltage dip, in a variable manner for the duration of the voltage dip,absorbing the active power necessary.
 9. Procedure for injectingreactive power with a device according to claim 1 onto a MV network towhich one or various electrical power generation machines are connectedwhen a voltage dip occurs on said network due to a mono-phase, two-phaseor three-phase fault, characterised in that the aforementionedmonitoring performed by the detection circuit (10) allows a voltage dipto be detected in a time of less than or equal to 1 millisecond. 10.Procedure for injecting reactive power with a device according to claim1 onto a MV network to which one or various electrical power generationmachines are connected when a voltage dip occurs on said network due toa mono-phase, two-phase or three-phase fault, characterised in that thevariation of the injected reactive power performed by the controlcircuit (20) takes place in time intervals of a less than or equal to 20milliseconds.
 11. Procedure for injecting reactive power with a deviceaccording to claim 1 onto a network to which one or various electricalpower generation machines are connected when a voltage dip occurs onsaid network due to a mono-phase, two-phase or three-phase fault,characterised in that said machines are wind turbines.